122 OXYTOCIN-INDUCED PROLONGED DIOESTRUS DOES NOT DOWNREGULATE THE EXPRESSION OF PROSTAGLANDIN-ENDOPEROXIDASE SYNTHASE 2 OR OXYTOCIN RECEPTOR IN EQUINE ENDOMETRIUM

Abstract

Lifespan of the corpus luteum is regulated through the release of PGF2α from the endometrium. The endocrine factors regulating the timed release of prostaglandin during luteolysis and the abrogation thereof during pregnancy is only partially understood in the mare. Vanderwall et al. (2007 J. Am. Vet. Med. Assoc. 231, 1864–1867) reported that twice-daily administration of oxytocin during early dioestrus results in prolongation of luteal function. We hypothesised that the aforementioned repeated administration of oxytocin during early dioestrus results in downregulation of oxytocin receptor transcript while prostaglandin-endoperoxide synthase 2 transcript abundance is unaffected. The current study, therefore, compared the expression oxytocin receptor (OXTR) and prostaglandin-endoperoxide synthase 2 (PTGS2) in endometrial samples obtained 16 days after ovulation from nonpregnant mares (n = 5), pregnant mares (n = 5), and mares displaying prolonged dioestrus after the repeated administration of oxytocin (n = 6). Prolonged dioestrus was observed in all mares treated with oxytocin, evident by plasma progesterone levels above 6 ng mL^–1 at the time of sample collection. Using quantitative real-time PCR analysis, PTGS2 showed a 4.37-fold lower abundance in pregnant v. nonpregnant mares (P = 0.009), while OXTR abundance was not affected by pregnancy status. Repeated administration of oxytocin did not lower expression levels of PTGS2 or OXTR when compared to levels observed in nonpregnant mares. As hypothesised, PTGS2 abundance was not affected by the treatment with oxytocin. Furthermore, we failed to prove the hypothesis that repeated administration of oxytocin leads to lowered expression of OXTR 16 days after ovulation in mares displaying prolonged dioestrus following repeated administration of oxytocin. A regulation of the oxytocin receptor at the post-transcriptional level, such as uncoupling of the receptor from its G-protein or possibly phosphorylation of the receptor, cannot be excluded and might account for the observed prolongation of luteal lifespan.